# Custom views [Define custom database views over onchain data]

Ponder supports custom [PostgreSQL views](https://www.postgresql.org/docs/current/tutorial-views.html) defined using Drizzle.

:::info
Custom views are not to be confused with the **views schema** pattern, an advanced feature that enables direct SQL queries. [Read more](/docs/production/self-hosting#views-pattern) about the views schema pattern in the self-hosting guide.
:::

## Usage

To define a view, use the `onchainView` function and write a query using Drizzle that references other tables or views in `ponder.schema.ts`.

```ts [ponder.schema.ts]
import { onchainTable, onchainView, count } from "ponder";

export const pets = onchainTable("pets", (t) => ({
  id: t.text().primaryKey(),
  name: t.text().notNull(),
  owner: t.text().notNull(),
}));

export const petLeaderboard = onchainView("pet_leaderboard").as((qb) =>
  qb
    .select({
      ownerName: pets.owner,
      petCount: count().as("pet_count"),
    })
    .from(pets)
    .groupBy(pets.owner)
);
```

Each view _must_ be a named export from the `ponder.schema.ts` file. The build step ignores views that are not exported.

## When to use custom views

Custom views are particularly useful in two common scenarios.

1. **Customize the GraphQL API**. With views, you can add custom fields to the GraphQL API without adding and populating an entire table.
2. **Move data processing from indexing-time to query-time**. By moving transformation logic to the query layer, views can simplify the project as a whole and help speed up lengthy backfills.

## Limitations

Custom views do not have a primary key constraint, which leads to several important limitations.

1. **Store API disabled**. The indexing function store API cannot access custom views. However, you can query custom views within indexing functions using raw SQL.
2. **No GraphQL singular query fields**. The GraphQL API does not include singular query fields for custom views.
3. **No GraphQL cursor pagination**. The GraphQL API includes plural query fields for custom views that support offset pagination, but do not support cursor pagination.

## Performance

Custom views are a useful tool to simplify indexing logic and provide a richer schema, but they are not magic. Each query against a custom view re-executes the stored `SELECT` statement.

To avoid performance issues, be sure to check the query plan for each custom view query and add database indexes on the underlying tables as appropriate.

## Examples

Here's an example of an `hourlyBucket` view from an ERC20 indexer which uses a `GROUP BY` query to aggregate transfer volumes over a time interval.

```ts [ponder.schema.ts]
import { onchainTable, onchainView, index, sql, sum, count } from "ponder";

export const transferEvent = onchainTable("transfer_event", (t) => ({
  id: t.text().primaryKey(),
  amount: t.bigint().notNull(),
  timestamp: t.integer().notNull(),
  from: t.hex().notNull(),
  to: t.hex().notNull(),
}));

export const hourlyBucket = onchainView("hourly_bucket").as((qb) =>
  qb
    .select({
      hour: sql`FLOOR(${transferEvent.timestamp} / 3600) * 3600`.as("hour"),
      totalVolume: sum(transferEvent.amount).as("total_volume"),
      transferCount: count().as("transfer_count"),
    })
    .from(transferEvent)
    .groupBy(sql`FLOOR(${transferEvent.timestamp} / 3600)`)
);
```
